This invention relates to satellites, particularly, using the global positioning system (GPS) for attitude control.
Global positioning system are extraordinarily accurate, employing a constellation of GPS satellites that transmit signals which a GPS receiver uses to determine position, in earth coordinates, and altitude. As explained in my U.S. Pat. No. 5,959,576, the attitude of an orbiting satellite can be regulated by communicating with and targeting one or more of the GPS satellites, a process that involves determining the elevation and azimuth to the GPS satellites and transforming earth-based GPS coordinates to space coordinates and using the on-board attitude control system, inertial measurement unit (IMU) control and momentum devices with thrusters.
Any use of GPS receivers with attitude capability requires a xe2x80x9cAttitude initializationxe2x80x9d sequence, the interval before the GPS receiver has resolved cycle ambiguity for accurate attitude determination. Special problems can arise, however, when a vehicle is launched from another vehicle in space, for instance CRV (crew recovery vehicle) launched from a space station to rapidly return a crew to earth. When the satellite is launched it may not be oriented with the GPS antennas pointed along zenith, and consequently the on-board GPS receivers cannot properly initialize attitude.
An object of the present invention is to provide a technique for adjusting to the satellite""s attitude so that the proper number of GPS satellites are received to initialize the on-board GPS receivers.
According to the invention, when the satellite is launched the GPS satellites that should be within the antenna""s filed of view (potential satellites) should be visible, are known from either an on-board processor or up linked data from a ground-based control. Using the GPS satellites that should be visible (the potential satellites) and those that are actually being tracked, an antenna bore-sight orientation is determined using the tracked GPS satellites position and the position, velocity and time of the spacecraft. The result is an antenna bore-sight estimation. Based on this estimation, the satellite is rotated (e.g. 90 degrees relative to the antenna bore sight) bringing into view other GPS satellites from which a second bore-sight estimation is obtained. A good estimate of the 3-axis attitude is obtained from these two vector observations plus the knowledge that the antenna bore-sight vector is aligned along the spacecraft Z axis. From these factors spacecraft""s attitude is ascertained and then an attitude control system, employing an inertial measurement unit and thrusters or momentum devices, is controlled to reoriented satellite so that the bore-sight vector points to the zenith. The desired attitude or attitude change to align the GPS antennas with zenith is determined on-board and appropriate commands are provided to the attitude control to cause the thrusters or momentum devices to change the satellite attitude to the desired attitude where the GPS receiver antennas are properly oriented, along zenith, to allow full GPS receiver initialization.
Objects, benefits and features of the invention will apparent to one of ordinary skill in the art from the drawing and description.